U.S. Pat. No. 4,573,402, assigned to the assignee of the present invention, discloses a system for treating a web of material such as paper by selectively treating the surface of a rotating calender roll. Heater packs are arranged in a row axially along and in proximity to the calender roll, and by controlling the power level to the heater pack in each zone, the calender roll is selectively heated.
Each heater pack is composed of two or more heating coils. In a preferred embodiment of the patent, the heater pack has two heating elements or coils mounted side by side and connected electrically in series.
Commercial modifications of the foregoing include as many as twelve coils in a single heater pack which can provide an output power level or heating effect of six kilowatts. These multiple coil packs also have been connected in series. A failure of one of the coils in the heater pack, of course, would cause the entire pack to fail and affect that proximate zone or portion of the web material or paper being manufactured. It would also necessitate an immediate replacement. Adjacent heater packs could have their power output or level raised to compensate but this is generally not satisfactory.